Co-packaged control circuit, transistor and inverted diode

ABSTRACT

A copackaged electronic device comprises a diode device having an anode coupled to a drain electrode of a switching device and a cathode capable of being coupled to an external circuit. The switching device may be controlled by an integrated circuit mounted on a source electrode of the switching device and electrically connected such that the integrated circuit is capable of controlling switching of the switching device. For example, the device is used in a power factor correction circuit. The diode device comprises at least one inverted diode having a solderable anode and a wire-bondable cathode.

RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/796,744, filed Mar. 9, 2004 to Stephen Oliver and Hugh D.Richard, entitled “Co-packaged Control Circuit, Transistor and InvertedDiode”, which is a continuation-in-part of U.S. application Ser. No.10/377,683 to Stephen Oliver and Hugh D. Richard, entitled“Semiconductor Package for Series-Connected Diodes,” filed Feb. 28,2003, which claims the benefit of U.S. Provisional Application No.60/408,519, filed Sep. 4, 2002, entitled “Tandem Diode Package with OneFlip Chip.” The disclosures of Ser. No. 10/377,683 and 60/408,519 areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The field of the invention is electronic packaging of switching device.Specifically, a diode is copackaged with a power switch and controlcircuitry.

BACKGROUND OF THE INVENTION

As shown in FIG. 1, a combined chip, consisting essentially of a powerswitch 10, such as a MOSFET, and an integrated circuit control chip 4mounted directly on a source electrode 13 of the power switch 10 by anadhesive 191, is known. In the example shown in FIG. 1, the adhesive 191is an electrically conductive adhesive that also serves to couple thebottom electrode 5 of the control chip 4 directly to the sourceelectrode 13 of the power switch 10. Alternatively, it is known to usean adhesive 191 that is electrically insulating, repositioning theelectrode 5 to the same surface of the control chip 4 as the otherelectrodes 7, 9. Then, the electrode 5 must be electrically coupled tothe source electrode 13 by any conventional process, such as by a wirebonding process. Such a combined chip may be packaged by surfacemounting the drain electrode 11 of the MOSFET 10 on a printed circuitboard 6 (a contact pad of which is partially shown), electricallyconnecting the drain of the MOSFET 10 to other electronic components bywire traces, for instance. However, a disadvantage of mounting aconventional combined chip directly to a PCB is that thermal managementof the power switch 10 is limited by the presence of the PCB adjacent tothe combined chip.

Also, conventional power factor correction circuits connect a separatelypackaged power switch with a separately packaged diode, such that thedrain electrode of the power switch, such as a MOSFET 10, iselectrically connected to the anode of the diode. However, such anarchitecture requires inventory and supply management, thermalmanagement and electrical contacts for each of the separately packagedpower devices, as both the power switch and the diode produce heat.Also, separately packaging the components and integrating the componentsin an electronic circuit requires a substantial amount of space formaking electrical connections between the separately packaged componentparts.

In addition, a conventional diode die is configured to have a wirebondable anode electrode on one side of the diode die and a solderablecathode electrode on the opposite side of the diode die. The anodeelectrode is wire bonded to one pin of a lead frame. The cathodeelectrode, which is solderable, is soldered to a copper pad of the leadframe, and the copper pad is electrically connected to another pin ofthe lead frame. Then, the pad, the diode die and a portion of the pinsare packaged by encapsulation, such as in an epoxy resin encapsulant, toprotect the diode die, allowing the packaged diode die to be connectedto an external electronic circuit, such as a printed circuit board(PCB).

SUMMARY OF THE INVENTION

A diode, a power switching device and an integrated circuit controllerfor the power switching device are copackaged in a discrete electronicpackage. For example, the copackaged electronic device may be used toreplace a plurality of discrete elements of a conventional alternatingcurrent (AC) to direct current (DC) converter using power factorcorrection (PFC).

In one embodiment, the copackaged electronic device comprises anencapsulated, discrete device, including a lead frame, a power switchingdevice, an integrated circuit controller for switching of the gateelectrode of the power switching device and a diode. The lead frame hasa pad having an electrically conducting mounting surface and a pluralityof leads for connecting to an external circuit, such as an AC to DCconverter. The power switching device, such as a MOSFET, has a firstelectrode (e.g. drain) on a first surface and two additional electrodes(e.g. source and gate) on an opposite surface. The first surface ismounted on the mounting surface of the pad. The integrated circuit (IC)controller is mounted on one of the two additional electrodes and may beoperably electrically coupled to the two additional electrodes by anyconventional process, such as soldering and/or wire bonding, forcontrolling switching of the power switching device. The diode may be aflip chip and may have its anode electrode electrically mounted to themounting surface of the pad. For example, the diode is laterally removedfrom the power switching device in a side-by-side arrangement. Leads ofthe lead frame are electrically coupled to the pad, the cathode of thediode, control electrodes of the IC controller and at least one of theadditional electrodes of the power switching device, such that thecopackaged electronic device may be operably connected to an externalcircuit.

In one example, a copackaged electronic device is packaged in a powerpackage, such as a TO220, D2pak, TO220FP or TO247, while including thecontrol circuit and the power switching device together with the diode.The number of discrete parts, the number of individual connectors, andthe number of wire bonds or wire traces between components are reducedcompared to conventional use of discrete components of an AC to DCconverter with power factor correction. For example, a through hole leadframe package may be mounted directly to a heat sink via an exposed backsurface of the lead frame pad, greatly improving and simplifying thermalmanagement of the heat generating power switching device and diode.

In one embodiment of the present invention, an integrated, electronicpackage has a plurality of diodes in series electrical contact mountedon an insulating layer on a portion of an electrically conductivecontact pad. Optionallyh, a common heat sink may be mounted to theopposite side of the lead frame providing thermal management of each ofthe heat-generating power devices. One diode may be a conventional diodewith an anode of a wire bondable material, such as an aluminum, and acathode of a solderable material, such as a solderable metal, e.g. acopper including, without limitation, solderable copper alloys. Thesolderable cathode may be joined to the electrically conductive contactpad by a thin layer of solder between the cathode and the lead frame,for example. The other diode may be an inverted diode. An inverted diodecomprises a cathode of a wire bondable material and a solderable anodeand, otherwise, may have a common semiconductor die architecture withthe conventional diode. For example, the two diodes are copacked with anintegrated circuit controller and/or a power switching device.

In one embodiment, a passivation layer surrounds the anode of theinverted diode. The passivation layer protects the termination structureof the inverted diode, allowing the anode to be directly joined to theelectrically conductive surface of a die pad. The passivation layerelectrically insulates the termination structure from the conductivelead frame.

One advantage of the copackaged device is that the size of a powerfactor correction circuit is reduced. Another advantage is that fewerwire bonds are required, reducing circuit resistance and inductance. Yetanother advantage is that the number of external leads for connecting toa circuit board, such as a PCB, is reduced compared to a conventionallymounted transistor, integrated circuit controller and diode. Yet anotheradvantage is that thermal management of the heat generating componentsis simplified and improved by copackaging the power switching device andthe diode.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an integrated circuit mounted on a switching device, whichis mounted on a pad (partially shown).

FIG. 2 shows an embodiment of the present invention.

FIG. 3 shows a circuit diagram of the embodiment shown in FIG. 2.

FIG. 4A shows a cross-section inverted diode having a solderable anodesurrounded by a passivation layer, according to the present invention.

FIG. 4B shows the cross-section of the inverted diode of FIG. 4A,mounted on a pad (partially shown).

FIG. 5 shows another embodiment of the present invention.

FIGS. 6A and 6B illustrate a diode with a passivation layer.

FIG. 7A illustrates use of diodes of FIGS. 6A and 6B in the embodimentof FIG. 5.

FIG. 7B illustrates diodes mounted on opposite sides of a die pad inanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to FIG. 2, one embodiment of a copackaged electronicdevice 2 comprises a single, 600 Volt (V) fast recovery diode 20electrically connected to the drain electrode 11 of a MOSFET 10 via anelectrically conductive pad 32 of a lead frame 30. The diode 20 islaterally removed from the MOSFET 10 on the mounting surface of the pad32 and the anode electrode 24 of the diode 20 is electrically mounted onthe mounting surface of the pad 32. For example, the diode 20 may be aflip chip diode (e.g. inverted), having the cathode electrode 24 of thediode 20 wire bonded 17 to a lead 12 of the lead frame 30. For example,a contact pad 40 electrically couples a wire bond 17 to the lead 12.Flip chip mounting of the diode 20 greatly simplifies the assemblyprocess and reduces the number of wire bonds required, which reduceswire bond resistance and inductance, improving the efficiency of thediscrete copackaged electronic device 2 of FIG. 2 compared to devicesusing a conventional diode.

In one embodiment, an integrated circuit 4 (IC) is mounted on the sourceelectrode 13 of the MOSFET 10 by a layer of electrically conductiveadhesive 19, such as solder, an adhesive tape or an epoxy, and iselectrically coupled to an electrode 5 on the bottom of the IC 4 to thesource electrode 13. Alternatively, the IC 4 may have all of itselectrical contacts 5, 7, 9 on the same major surface, the layer ofadhesive may be electrically insulating, and electrode 5 may beelectrically coupled to the source electrode 13 by any conventionalprocess, such as by a wire bond. For example, FIG. 1 shows an electricalcontact 7 on the IC 4 that is electrically coupled by a conventionalwire bond 17 to the gate electrode 15 that is used for controlling theswitching of the MOSFET 10. The IC 4 is also electrically coupled to thecontrol leads 14, 16 of the lead frame 30, as shown in FIG. 2, forexample. As shown in FIG. 3, the leads 14, 16 provide for coupling ofthe IC 4 to a power factor correction circuit 30, which provideselectrical signals or voltages to the IC 4, as is known in the art ofpower electronics.

The mounting surface 33 of the pad 32 of the lead frame 30 may be madeof any electrically conductive material, such as a homogenous metallayer. For example, the entire pad 32 is a metal, such as a copper or analuminum metal, which improves thermal heat transfer through the pad 32.In one example, the copackaged electronic device 2 is packaged using anencapsulant (not shown), such as an epoxy resin, to encapsulate theelectronic components, but leaving the back surface 35 of the pad 32exposed, allowing a heat sink (not shown) to be directly coupled to thepad 32. Thus, thermal management of the heat generating components, theMOSFET 10 and the diode 20, is greatly simplified compared to thethermal management required for use of conventional discrete components.

The wire bonds 17 may be made by any conventional process, such as aconventional wire bonding process using gold or an aluminum. The anodeelectrode 24 of the diode 20 is electrically coupled to the pad 32 ofthe lead frame 30. For example, an electrically conductive adhesive 19,such as a solder, an adhesive tape or a continuous, metal-filled epoxy,may be used to electrically couple the electrode 11 of the MOSFET 10 andthe anode electrode 24 of diode 20 to the mounting surface 33 of the pad32 of the lead frame 30. Preferably, the anode electrode 24 of the diode20 is solderable, and the thermal resistance of the pad 32, the anodeelectrode 24 and the layer of solder 19 is reduced, allowing efficientheat removal from the diode 32.

In one embodiment, a copackaged device 2 is fabricated by manufacturinga conventional MOSFET 10 and a flip chip diode 20, such that the cathode22 of the diode 20 is made of an aluminum, and the anode electrode 24 ismade for joining to the mounting surface 33 of the pad 32 of the leadframe 30. Preferably, the anode electrode 24 is made of a solderablemetal, such as copper or a copper alloy, and the anode electrode 24 issurrounded by a passivation layer 26, which may be made of an insulatingepoxy, for example. The passivation layer 26 shields the terminationstructure 28. For example, during joining of the anode electrode 24 tothe lead frame pad 32, the passivation layer 26 shields the terminationstructure 28 from contacting solder 2 or electrically conductiveadhesive 2.

In one embodiment, both the drain 11 of the MOSFET 10 and the anodeelectrode 24 of the diode 20 are mounted on the same surface 33 of thepad 32 of the lead frame 30 in a side-by-side configuration, allowingthe back side 35 of the pad 32 to remain exposed after the electronicdevices 4, 10, 20 are encapsulated by an encapsulant. The IC 4 may bemounted on the MOSFET 10 either after the MOSFET 10 is mounted on thepad 32 or prior to mounting the MOSFET 10 on the pad 32. In oneembodiment, the IC 4 is mounted on the MOSFET 10, and then both theMOSFET 10 and the diode 20 are joined to the pad 32 simultaneously in acommon soldering step.

For example, contacts 3, 7, 9 on the surface of the IC 4 are then wirebonded to the gate electrode 15 of the MOSFET 10 and to external leads14, 16, as shown in FIG. 2. Input leads 14, 16 may be connected to anexternal circuit board (not shown) to provide voltage signals for the IC4 to control the switching of the MOSFET. Output lead 12 outputs theresulting output voltage of the copackaged device, and the source lead Sand drain lead D are used as external connections to the source anddrain electrodes S, 11 of the MOSFET 10. Other configurations ofcontacts and leads may be used to integrate the copackaged device in apower factor correction circuit, for example, as is known in the art.

Preferably, by copackaging the IC 4, MOSFET 10 and diode 20 asdescribed, the copackaged device 2 is capable of being housed in adevice having the same form as a conventional diode package, such as aD2-pak, TO220 or TO247. For example, a 5-pin TO220 standardconfiguration may be fabricated by enclosing the IC 4, MOSFET 10 anddiode 20, as shown in FIG. 2 within a resinous encapsulant. Preferably,the back surface 35 of the pad 32 remains exposed for mounting a heatsink (not shown) to the pad 32, enhancing heat transfer by limiting thethermal resistance between the heat generating components 10, 20 and theheat sink. Copackaging of these heat-generating power devices 10, 20simplifies thermal management by using a common heat sink, for example.Alternatively, a 6-pin or 7-pin package may be used, offering additionaloutput pins for connecting with an external electrical circuit.

As shown in FIG. 5, two 300 V diodes 120, 121 are connected in series ina tandem diode package 220, creating a single 600 V fast diode forcontinuous mode power factor correction. As shown in FIG. 5, a tandemdiode circuit 220 comprises a first diode 120, an inverted diode 121, adiode contact pad 129, a pin contact pad 40 and an electrical lead 12.One diode 120 is wire bonded from its anode 24 to a mounting surface ofa conductive die pad 30. The die pad 30 is integrally attached to acentral pin D. An inverted diode 121 has a cathode 122 that is wirebonded to a pin contact pad 40. The contact pad 40 is integrallyattached to an electrical lead D. Alternatively, lead D may be separatedfrom contact pad 40, and the contact pad 40 may be coupled to the lead Dby any conventional process, such as wire bonding. Thus, pin D iselectrically coupled to the anode 24 of one diode 120 and lead 12 iselectrically coupled to the cathode 122 of the tandem diode package 220,such that the tandem diode package 220 may be connected in a circuitwith the copackaged MOSFET 10 and IC 4.

The contact pad 129 may be made of any electrically conductive material.Preferably, the contact pad 129 is a metal, such as a copper or analuminum metal and is insulated from the mounting surface 33 of the leadframe 30 by an insulating layer 139, such as an insulating adhesivelayer or ceramic layer. Wire bonding may be completed by anyconventional process, such as a conventional wire bonding process usinggold wires. Each of the diodes 120, 121 are electrically bonded to thecontact pad 129. For example, a solder or conductive adhesive, such as acontinuous, metal-filled epoxy, may be used to make an electricalconnection between the cathode of the first diode 120 and the anode 122of the inverted diode 121 via the contact pad 129.

In one embodiment, the tandem diode package 220 is fabricated bymanufacturing one conventional diode 120 and one inverted diode 121. Theconventional diode 120 may be prepared by any conventional process. Thesemiconductor die 128 of inverted diode 121 may be made by the sameprocess; however, the cathode 124 is made of a material for wire bondingbetween the cathode 124 and the contact pad 40, and the anode 122 ismade for joining to the tandem diode contact pad 129. Preferably, thecathode 124 is of an aluminum, such as a wire bondable aluminum alloy.Preferably, the anode 122 is made of a metal and is surrounded by apassivation layer 127, which may be made of an insulating epoxy, forexample. The passivation layer 127 protects the termination structure126, during joining of the anode 122 to the tandem diode contact pad129, from contacting the electrically conductive material adhering theanode 122 to the contact pad 129. More preferably, the metal of theanode 122 of the inverted diode 121 is of a solderable metal.

For example, both semiconductor diode dies 128 may be made on the samewafer by applying and patterning anodes, cathodes, terminationstructures and passivation layers as appropriate on the anode andcathode sides of each semiconductor device. The semiconductor diode diesare then separated, such as by sawing or laser cutting the dies from thewafer. The conventional diode 120 has its cathode 22 soldered to thecontact pad 129, and the inverted diode 121 has its anode 122 solderedto the contact pad 129. Preferably, both of the diodes 120, 121 aremounted on the same surface of the lead frame 30 in the same die bondingstep. Then, both diodes 120, 121 are wire bonded to their respectivewire bonding points. Packaging of the tandem diode structure 220 is thencompleted as previously addressed. Alternatively, the tandem diodes 120,121 may be mounted first on a tandem diode contact pad 139, and then thecontact pad may be mounted on the lead frame 30 prior to wire bonding ofthe diode electrodes.

A plurality of diodes 120, 121 may be connected in series by thisprocess, forming a very fast, high voltage diode package. At highvoltages, a tandem diode package 220 has a faster reverse recovery timethan a single diode of the same rating. For example, two fast diodes120, 121 connected in series on a single contact pad 139 may be housedin a standard power diode package, such as a TO220, D2-pak, TO220FP orTO247, without any need of internal insulation. For example, parallelpins 144, 146, 148 extend from one side of the lead frame 130 as shownin FIGS. 7A.

As shown in FIGS. 7A and 7B, tandem diodes 120, 121 may be packaged inseries by placing two identical lead frames 30 back-to-back andseparated by an insulating layer 130. For example, the tandem diodes120, 121 are then mounted on one surface and wire bonded to contact pads140, 141, which may be wire bonded to electronic devices on the oppositesurface of the lead frame 30 by wire bonds 153 as shown in FIG. 7A. Inthis example, the surface of the lead frame 30 serves as a contact pad139, such that the tandem diodes 120, 121 are connected in series.

For example, the lead frame 30 may comprise a composite structure havingan electrically conducting top surface 137 and an electricallyconducting bottom surface 138 separated by an electrically insulatinglayer 130 sandwiched between the top surface and the bottom surface. Forexample, the electrically insulating layer 130 may be a thermallyconductive layer of a phase change sheet, a tape, an epoxy, a dielectriccoating, a boron nitride layer, a silicone grease or a silicone/boronnitride composite.

In one embodiment, the lead frame 30 has a plurality of diodes,including at least one inverted diode with a passivation layer aroundthe anode, mounted in series on only one surface of the lead frame 30.The opposite surface of the lead frame 30 may be exposed for dissipationof heat. For example, the opposite surface thermally connects to a heatsink (not shown), which extracts heat from the diode packageefficiently. In this alternative embodiment, the MOSFET 10 and IC 4 maybe copackaged in a separate, discrete package that is mounted on thesame heat sink as the diode package. For example, the two discreteelectronic packages may be mounted side-by-side or on opposite sides ofa common heat sink.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by theexamples herein, but only by the claims themselves.

1. A semiconductor package, comprising: an electrically conductive padhaving a top surface and a bottom surface; a first diode having a firstanode and a first cathode opposite of the first anode, the first cathodebeing mounted and electrically connected to the top surface of theconductive pad; a second diode having a second cathode and a secondanode opposite of the second cathode, the second anode being mounted andelectrically connected to the top surface of the conductive pad, wherebysaid first diode and said second diode are electrically connected inseries through said conductive pad; a first lead electrically connectedto said first anode; and a second lead electrically connected to saidsecond cathode.
 2. The semiconductor package of claim 1, furthercomprising: another conductive pad, and an insulating body sandwichedbetween the conductive body and the another conductive body.
 3. Thesemiconductor package of claim 2, wherein the insulating body includes aphase change sheet, or a tape, or an epoxy, or a dielectric coating, orboron nitride layer, or a silicone grease, a silicone/boron nitridecomposite.
 4. The semiconductor package of claim 2, further comprisinganother semiconductor device disposed on said another conductive pad. 5.The semiconductor package of claim 4, wherein the another semiconductordevice is a power MOSFET.
 6. The semiconductor package of claim 4,wherein the another semiconductor device is electrically connected to atleast one of the diodes.
 7. The semiconductor package of claim 4,wherein the another semiconductor device is electrically connected to acorresponding lead.
 8. The semiconductor package of claim 1, wherein thebottom surface of the conductive pad is exposed.
 9. The semiconductorpackage of claim 1, wherein said package conforms to the TO-220standard.
 10. The semiconductor package of claim 1, wherein said packageconforms to the D2-pak standard.
 11. The semiconductor package of claim1, wherein said package conforms to the TO220FP standard.
 12. Thesemiconductor package of claim 1, wherein said package conforms to theTO247 standard.
 13. The semiconductor package of claim 1, wherein saidleads extend in the same direction.
 14. The semiconductor package ofclaim 13, wherein said leads are parallel one another.